Recent years have seen development of a light-emitting device that combines (i) a semiconductor light-emitting element such as a light-emitting diode (LED) with (ii) a wavelength converting member that converts excitation light from the semiconductor light-emitting element into fluorescence (for example, a member containing fluorescent material particles dispersed in resin). The above light-emitting device, which is advantageously compact and consumes less power than an incandescent lamp, is in practical use as a light source for any of various display devices and illumination devices.
Patent Literature 1 discloses a light-emitting device that outputs pseudo-white light. This light-emitting device combines a blue LED with a fluorescent material (wavelength converting member) that is excited by blue light from the blue LED and that converts the wavelength of the blue light to emit yellow light.
Recently, studies have been conducted of using, for example, a semiconductor laser, which has a light density higher than that of a blue LED or the like, as an excitation-light source for the above light-emitting device. Further, studies have also been conducted of using, as excitation light, light having a wavelength shorter than that of blue light.
Such arrangements are, however, problematic in that resin in which fluorescent material particles are dispersed is degraded by heat or light. To solve this problem, techniques have been proposed of using glass as a material in which to disperse fluorescent material particles.
Patent Literature 2, for example, discloses a wavelength converting member including fluorescent material particles (which are made of a material such as an oxide, a sulfide, an oxysulfide, a halide, or an aluminate) dispersed in glass. Patent Literature 2 mentions ZnO—B2O3—SiO2-based glass as a material suitable for the glass.
In Patent Literature 2, the glass material has a composition selected so that the glass can be fired at a temperature within a range of relatively low temperatures. Specifically, the glass material has a composition selected so that the firing temperature for glass is 750° C. or lower. This selection intends to prevent fluorescent material particles dispersed in glass from being degraded by heat when the glass is fired.
The glass material used in Patent Literature 2 is, however, problematically low in transparency and unstable thermally and chemically as compared to silica glass (which is a glass that does not contain any metallic element other than Si [such as Zn and Ba] and that is made of SiO2 only).
Regarding the above point, Non Patent Literature 1 discloses an arrangement in which fluorescent material particles made of an oxynitride fluorescent material are dispersed in silica glass. In Non Patent Literature 1, the silica glass is prepared through a sol-gel process.
In Non Patent Literature 1, the fluorescent material particles are made of an oxynitride fluorescent material, which is excellent in thermal and chemical stability. This allows silica glass in which the fluorescent material particles are dispersed to be fired at a high firing temperature of 1050° C.
Silica glass is, as described above, high in transparency and excellent in thermal and chemical stability. Non Patent Literature 1 thus allows production of a wavelength converting member that is high in luminous efficiency and excellent in durability.
Note that in Non Patent Literature 1, the firing temperature for silica glass is a high temperature of 1000° C. or higher. This makes it necessary to select, as the fluorescent material particles to be dispersed in silica glass, fluorescent material particles that are not degraded thermally in the atmosphere even at a temperature of 1000° C. or higher.
Among various fluorescent material materials in practical use, however, the α-SiAlON fluorescent material, disclosed in Non Patent Literature 1, is the only material that satisfies the above condition. Stated differently, the technique disclosed in Non Patent Literature 1 unfortunately allows only one color for a fluorescence emitted by fluorescent material particles (that is, α-SiAlON fluorescent material particles) dispersed in silica glass.
This means that the technique disclosed in Non Patent Literature 1 fails to make it possible to (i) disperse in silica glass a plurality of kinds of fluorescent material particles which kinds emit respective fluorescences having different colors and (ii) mix such fluorescences emitted by the respective kinds of fluorescent material particles and having different colors. The technique disclosed in Non Patent Literature 1 thus problematically has a low degree of freedom in designing the color of light emitted by a wavelength converting member.
The wavelength converting member of Non Patent Literature 1 has a low degree of freedom in designing the color of light emitted by the wavelength converting member. Thus, in a case where the wavelength converting member is combined with an excitation-light source to produce a light-emitting device, the light-emitting device problematically has a degraded color rendering property. Further, in a case where the light-emitting device is included in a display device, the display device problematically has decreased color reproducibility.
Non Patent Literature 2 and Patent Literature 3 each disclose an arrangement in which (i) a film of an oxynitride fluorescent material is formed on a substrate to form a fluorescent material film and (ii) the fluorescent material film is then bonded to the substrate with use of silica glass.
In Non Patent Literature 2 and Patent Literature 3, the firing temperature for silica glass is approximately 500° C., which is lower than the firing temperature of Non Patent Literature 1. The respective techniques of Non Patent Literature 2 and Patent Literature 3 thus each allow a wide range of options, as compared to the technique of Non Patent Literature 1, for the material of fluorescent material particles to be excited by blue light serving as excitation light.
Specifically, the respective techniques of Non Patent Literature 2 and Patent Literature 3 may each use, as a material of the fluorescent material particles, (i) an oxynitride fluorescent material such as a β-SiAlON fluorescent material or (ii) a nitride fluorescent material such as a CaSiAlN3 fluorescent material (CASN-based fluorescent material).
According to the respective techniques of Non Patent Literature 2 and Patent Literature 3, however, the wavelength converting member (light-emitting section) is limited in shape to a film. This limitation leads to a low degree of freedom in the shape of the wavelength converting member and also to an excessive amount of fluorescent material particles to be contained in the wavelength converting member. The fluorescent material particles may, for example, be contained in the wavelength converting member in an amount as large as 30% or greater by volume.
In a case where a wavelength converting member containing fluorescent material particles excessively as described above is included in a light-emitting device, those fluorescent material particles cause light to be scattered excessively in the wavelength converting member, with the problematic result of decreased efficiency of extraction of light from the wavelength converting member (stated differently, decreased efficiency of excitation light conversion by the wavelength converting member).
In the case where the efficiency of extraction of light from the wavelength converting member is decreased, a light-emitting device that combines the wavelength converting member with an excitation-light source problematically has decreased luminous efficiency.
Patent Literature 4 discloses firing silica glass at a low firing temperature through a sol-gel process. Specifically, in Patent Literature 4, sulfide fluorescent material particles are dispersed in silica glass, and this silica glass is fired at a firing temperature of 150° C.